1. Field of the Invention
The present invention relates to a semiconductor laser suitable for a light source in optical fiber communication, and a method of fabricating the same.
2. Description of the Related Art
Conventionally-used semiconductor lasers are such as those having a non-doped quantum dot active layer formed on an n-type substrate. In recent years, investigation into semiconductor lasers having formed therein a p-type-doped or p-type-modulation-doped (the both will generally be referred to as “p-type-doped”, hereinafter) quantum dot active layer, in place of the non-doped quantum dot active layer. The latter semiconductor laser is increased in the differential gain and improved in the modulation characteristics, as compared with the former. It is also made possible to readily suppress the temperature dependence of laser light. The latter semiconductor laser is therefore considered as promising as a light source for metro/access optical fiber communication.
Structures of the conventional semiconductor laser will be explained. The structures of the conventional semiconductor laser are roughly classified into ridge structure and high mesa structure. FIG. 4 is a sectional view showing a conventional ridge-structured semiconductor laser, and FIG. 5 is a sectional view showing a conventional high-mesa-structured semiconductor laser.
The ridge-structured semiconductor laser has, as shown in FIG. 4, an n-type cladding layer 102 formed on an n-type GaAs substrate 101, and has a quantum dot active layer 103 formed further thereon. The quantum dot active layer 103 has a non-doped intrinsic GaAs layer 103c, a p-type GaAs layer 103d and a non-doped intrinsic GaAs layer 103e stacked therein. InAs quantum dots 103b are formed on the non-doped intrinsic GaAs layer 103e. Another intrinsic GaAs layer 103c, another p-type GaAs layer 103d and another intrinsic GaAs layer 103e are stacked so as to cover the InAs quantum dots 103b. 
A p-type cladding layer 104 is formed on the center portion of the quantum dot active layer 103, and a p-type contact layer 105 is formed further thereon. Still further thereon, a SiO2 film 106 is formed so as to cover the quantum dot active layer 103, the p-type cladding layer 104 and the p-type contact layer 105. The SiO2 film 106 has an opening formed therein, so as to allow the center portion of the p-type contact layer 105 to expose therein, and an electrode 107 is formed in the opening. The n-type GaAs substrate 101 also has an electrode 108 formed on the back surface thereof.
On the other hand, in the high-mesa-structured semiconductor laser shown in FIG. 5, the quantum dot active layer 103 is formed on the center portion of the n-type cladding layer 102. On the quantum dot active layer 103, the p-type cladding layer 104 and the p-type contact layer 105 are formed.
These semiconductor lasers having the p-type doped quantum dot active layer has been under investigation, but suitability for the practical use has not been reported yet.
The present inventors practically confirmed the operations, and found that the ridge-structured semiconductor laser was incapable of high-speed modulation operation at a speed exceeding 10 Gb/s. The high-mesa-structured semiconductor laser was found to generate current component not contributive to laser oscillation due to surface non-radiative recombination, and to thereby degrade the reliability.
Related arts are disclosed in Patent Document 1 (Japanese Patent Application Laid-Open No. 2004-63957), Patent Document 2 (Japanese Patent Application Laid-Open No. 2003-309322), Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-144379), Patent Document 4 (Japanese Patent Application Laid-Open No. 9-18086), Non-Patent Document 1 (Proceedings of the 62nd Device Research Conference, presentation No. VI.C-4), Non-Patent Document 2 (Japanese Journal of Applied Physics Vol. 43, No. 8B, 2004, pp. L1124-L1126), and Non-Patent Document 3 (Proceedings of 30th European Conference on Optical Communication, presentation No. Th4.3.4).